A fuel pipe for automotive use is required under U.S. laws to have a guaranteed lifetime of 15 years or 150,000 miles. Fuel pipes made of stainless steel (SUS436L: 17Cr-1.2Mo) as materials have already been commercialized.
Automobiles which operate in the North American region are exposed to de-icing salt spray environments, so the materials which are used for the fuel pipes are required to have excellent salt spray corrosion resistance. In the past, SUS436L has been applied. However, due to the recent soaring prices of resources, demands have arisen for reducing the material costs. SUS436L contains the expensive Mo in about 1%, so even with just replacement by the Mo-free AISI439 steel (17Cr), a large effect of reduction of costs is obtained.
However, if cutting the alloying elements of a material, the corrosion resistance deteriorates. Therefore, the weak points due to lowering of the grade of the material require compensation by other methods.
The locations of a fuel pipe where corrosion is a concern are the crevice structures at the outer surface side of the fuel pipe which is exposed to a salt spray environment. In the past, as a means for improving the salt spray corrosion resistance of a crevice part in a crevice structure, coating by cationic electrodeposition has been used.
PLT 1 discloses a method of production which uses SUS436 pipe as a material and uses projection welding to assemble a fuel pipe and coats it by cationic electrodeposition. However, according to the findings of the inventors, with this art, the corrosion proofing cannot be said to be complete even with SUS436. Therefore, when using a lower grade material, it cannot be deemed that a sufficient corrosion proofing effect can be obtained by this art.
PLT 2 discloses the art of using SUS436 as a material so as to assemble a fuel pipe and coating the fuel pipe by electrostatic coating to prevent crevice corrosion.
PLT 3 shows the art of coating stainless steel fuel pipe by a chip-resistant coating so as to secure a sufficient corrosion proofing property even with chipping.
However, these arts involve higher coating costs than electrodeposition coating. On the other hand, the insides of crevices cannot be coated, so a sufficient corrosion proofing effect cannot be obtained.
Various corrosion proofing methods aside from coating have also been proposed.
PLT 4 discloses the art of placing sacrificial positive electrodes of zinc at the locations where passivation films are damaged by welding, brazing, plastic working, etc. in the assembly of stainless steel fuel pipes or at crevice locations and making use of sacrificial corrosion.
However, placing zinc at all of the locations where corrosion is a concern is troublesome and time-consuming. Further, zinc is an expensive metal. Furthermore, zinc is easily consumed in a salt spray environment, so the amount required soars. Therefore, in a fuel pipe, sacrificial corrosion cannot be said to be a realistic technique.